This invention relates to a novel electrodeionization module adapted to transfer ions in a liquid under the influence of a polar field, More specifically, this inventions relates to an electrodeionization apparatus adapted to purify aqueous liquids to effect the production of high purity water and to minimize scale formation.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. The most well-known processes include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration and ion exchange. A lesser known method is electrodeionization, occasionally mistermed filled cell electrodialysis.
The first apparatus and method for treating liquids by electrodeionization was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an apparatus and process for the removal of ions within a liquid mixture in a depleting chamber through a series of anionic and cationic membranes into a second volume of liquid in a concentrating chamber under the influence of an electrical potential which causes the preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transfer ions and carries them in concentrated form. The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic membranes. This ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membrane for the movement of ions.
The term "electrodeionization" refers to the process wherein an ion exchange material is positioned between anion and cationic membranes. The term "electrodialysis" refers to such a process which does not utilize ion exchange resins between the anionic and cationic membranes. Illustrative of other prior art attempts to use the combination of electrodialysis and ion exchange materials or resins to purify saline from brackish water are described in U.S. Pat. Nos. 2,794,770; 2,796,395; 2,947,688; 3,384,568; 2,923,674; 3,014,855 and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517 and 4,284,492.
In any membrane separation process where ions become concentrated there is the potential to exceed solubility limits and form scale. In particular, calcium carbonate scale, CaCO.sub.3, is formed when the levels of Ca.sup.2+ and CO.sub.3.sup.2- in water reach a solubility limit. EQU (1) Ca.sup.2+ +CO.sub.3.sup.2- &lt;=&gt;CaCO.sub.3, [Ca.sup.2+ ][CO.sub.3.sup.2- ]=K.sub.sp
Furthermore, the level of CO.sub.3.sup.2- in water is function of the pH of the water and the equilibrium with bicarbonate, HCO.sub.3.sup.--. ##EQU1##
By combining the equations above, the scale potential relative to calcium and bicarbonate concentrations and the pH of the water is defined. ##EQU2##
The potential for forming scale increases with an increase in calcium ion concentration, an increase in bicarbonate ion concentration or an increase in pH. In addition, the ion concentration increases when the electrodeionization module is operated to recover an increased percentage of incoming water as purified water.
Reactions at the electrodeionization electrodes and water splitting in the electrodeionization process can create significant shifts in the pH of the waste water stream. The reactions occurring at the electrodes are shown below. The generation of OH.sup.-- at the cathode creates an area of high scale potential.
______________________________________ Anode Reaction Cathode Reaction ______________________________________ (4) 2H.sub.2 O &gt;&gt; 4H.sup.+ + 4e.sup.- + O.sub.2 (5) 2H.sub.2 O + 2e.sup.- &gt;&gt; 2OH.sup.- + 2Cl.sup.- &gt;&gt; 2e.sup.- + Cl.sub.2 H.sub.2 ______________________________________
Water splitting within the diluting compartments is an additional source of OH.sup.-- within the electrodeionization module. In the concentrating compartment where OH.sup.- is entering through the anion membrane and especially along the surface of that anion membrane is were pH can become high resulting in areas with high risk of scale formation. Since pH is the negative log of the H.sup.+ concentration, what appears to be a small change in pH will have a significant impact to scale potential. For example an increase in one pH unit will increase the scale potential by a factor of 10.
Formation of scale within the electrodeionization module will result in a very high electrical resistance and blocked flow channels leading to a quick decline in water quality produced. Several methods or combination of methods presently are used to reduce the risk of scale in an electrodeionization module. In a first method water is pretreated before it enters an electrodeionization module to reduce the levels of Ca.sup.2+ and/or HCO.sub.3.sup.-- or to decrease pH thereby reducing the potential to form scale. For example, a properly functioning ion exchange softener will reduce Ca.sup.2+ levels by exchanging 2Na.sup.+ for the Ca.sup.2+ in the feed water. The solubility of Na.sub.2 CO.sub.3 is very high with almost no risk of scale formation. The softener is regenerated by treating the resin with high concentrations of NaCl. Although a very effective method to reduce scale potential, softening has had limited success with the electrodeionization product. Improper maintenance, increases in feed water Ca.sup.2+ levels, and/or increases in the volume of water treated results in high leakage of Ca.sup.2+ and subsequent scaling of the electrodeionization module. In addition, the added cost and the size of a softener resin tank and salt regeneration tank are not desirable and especially does not fit with the concept of the compact, reliable and easy to use approach desired for a smaller laboratory water system.
In a second pretreatment process, reverse osmosis (RO) is used to remove greater than 90 to 95% of the Ca.sup.2+ and HCO.sub.3.sup.- in the feed water thereby significantly reducing the potential to form scale. However, in locations where the Ca.sup.2+ and HCO.sub.3.sup.- levels are very high, (over 100 ppm of Ca.sup.+2 feeding the RO), enough Ca.sup.+2 and HCO.sub.3.sup.- pass the RO so that an electrodeionization module can still suffer from scale formation. In these locations the current technology is forced to use softening to pretreat the RO prior to electrodeionization.
A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745. The apparatus utilizes ion depleting compartments containing an ion exchange solid composition and a concentrating compartment which is free of ion exchange solid material. The electrodeionization apparatus includes two terminal electrode chambers containing an anode and a cathode respectively which are utilized to pass direct current transversely through the body of the apparatus containing a plurality of ion depleting compartments and ion concentrating compartments. In operation, the dissolved ionized salts of the liquid are transferred through the appropriate membrane from the ion depleting compartments to the ion concentrating compartments. The ions collected in the ion concentrating compartments are removed through discharge outlets and are directed to waste. The deposit of insoluble scale within the cathode compartment has been a problem associated with this process.
It has been proposed in U.S. Pat. No. 3,341,441, in an electrodialysis process, to reverse periodically the direction of current flow in which case, the electrode once serving as the cathode becomes the anode while the anode chamber becomes the cathode. The solution flowing through the anode chamber becomes acidic due to anodic electrolytic action, and the acid thus formed tends to dissolve a small portion of scale formed therein during the time the electrode was cathodic. In the process the flow is reduced or stopped and thus, the acid generated within the anode chamber is allowed to attain a sufficiently high concentration in the chamber so as to dissolve precipitated scale formed therein during the electrode's previous cathodic cycle and thereafter, reversing the polarity of the direct current is performed at periodic intervals. In a preferred form of the process, a third step is also employed comprising continuously flushing the cathode compartment with a sufficiently large volume of electrolyte solution to quickly remove any base generated therein. When the direct current is reversed, the ion depleting compartments become the ion concentrating compartments and the ion concentrating compartments become the ion depleting compartments. This process can be undesirable since a large volume of liquid being purified must be discharged to waste in a time interval immediately following voltage polarity reversal since the concentration of electrolyte in the newly formed ion depleting compartments is too high for a period of time to render the purity of the liquid product acceptable.
It has also been proposed in U.S. Pat. No. 4,956,071 to utilize voltage polarity reversal in an electrodeionization process in order to reduce scale formation. In the process, the voltage through the process is periodically reversed, typically every 15 to 20 minutes, with the voltage polarity in a given direction being approximately 50% of the time of process operation. With each voltage polarity reversal, the dilution compartments become concentration compartments and the concentration compartments become dilution compartments. As a result of the voltage polarity reversal, several valves are needed in the system for distributing the streams. Two valves typically are needed to direct the appropriate dilute stream to the final point of use. In addition, control means for these valves may be required to direct water to drain until acceptable purity levels are reached. In addition, one or two additional valves typically are used to control flow rates to electrode streams in order to optimize pH shifts and scale prevention. This patent discloses that an electrode spacer having an ion permeable membrane and positioned adjacent to anode and cathode optionally can be filled with ion exchange resin.
Additional electrodeionization apparatus are disclosed by U.S. Pat. Nos. 5,154,809; 5,308,466 and 5,316,637. U.S. Pat. No. 5,308,466 discloses, an electrodeionization apparatus utilizing concentration compartments containing ion exchange resin. The advantage provided by ion exchange resins in the concentrating compartments is improved performance and, specifically for improved removal or separation of highly charged, large highly hydrated, or weakly ionized species, silica, sulfate, calcium, heavy metals, and polar and ionized organics. The patent does not discuss the effect on scale formation as a result of utilizing resins in the concentrating compartments.
U.S. Pat. No. 4,226,688 discloses an electrodialysis apparatus including a cathode compartment and an anode compartment. A conductive slurry of carbon particles is continuously transferred between the electrode compartments at a rate of at least 1 ml/min/per cm.sup.2 of electrode area. Hydrogen produced at the cathode compartment is absorbed by the carbon particles and released at the anode compartment. Scale production and corrosion problems are reduced by this process. The process is undesirably complex in that it requires pumping, conduits and control apparatus.
Accordingly, it would be desirable to provide an electrodeionization process which minimizes or prevents scale formation. In addition, it would be desirable to provide such a process which does not require a complex piping, valving, pumping and control system for directing a newly produced dilute stream to a point of final use or to transfer scale reducing compositions between electrode components.